If I've got a bit of glass, and a scrap of the same glass, can I tell by measuring the viscosity (say how much it drapes over a form in a few minutes) at a given temperature how much cooler it needs to be to anneal?
The simple test for annealing temp is to measure the sag temp. To do this take a long slender piece of the glass and either prop it on supports at both ends or support one end between two fire brick. Bring the temp up slowly until it just begins to sag. [In reality, pretest to get a rough idea of the sag point, then put in the test piece, start about 20-40F below that temp, raise the temp 5 degrees and let it settle. If the glass does not move, raise it 5 more and hold, etc.] The sag and annealing temps are related to specific viscosities which are very hard to measure. The annealing temp is taken to be 90F or 50C below the sag temp. You should be able to hold at the annealing temp without sagging. All the thin glass should be held at that temp for an hour before starting down. Thick glass, like paperweights, should be held longer. See schedules in books. The idea is to work out the strains as quickly as possible without sagging the glass and then cool slowly enough not to introduce new strains.
Don't have books, have kiln :)
Do you have a book title that I can check out of the library?
Probably not at the library. Take a look at
I suspect I probably could - last book checked out "Huzel & Huang - Design of liquid propellant rocket engines". Interlibrary loan is fun.
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